This invention relates in general to a method of producing an activated type of alumina from aluminum dross. More particularly, it relates to utilizing the peculiar properties of the activated alumina, which result from the method used in recovering it from dross that is skimmed or otherwise separated from molten aluminum or aluminum alloys. This dross may float on the surface of molten aluminum or sink as a sludge therein and thus be removed by skimming, filtering through screens or filtering through granular filter beds.
Aluminum dross usually contains 50 to 80% metallic aluminum entrained in aluminum oxide, and much smaller quantities of aluminum nitride, aluminum carbide, alkali and alkaline earth oxides and halides, which are used as fluxes on the molten aluminum surface to decrease oxidation during melting or holding in a molten condition prior to casting. The use of dross or aluminous skimmings for feed to Hall cells for the electrolytic reduction of aluminum is as old as the Hall process. However, the nitrides and carbides are deleterious to Hall cell operation, because they segregate from the cryolite bath or flux and tend to accumulate on the carbon cathode. The nitrides in particular act as an electrical insulation layer on the carbon cathode, and make current densities uneven, so production of aluminum suffers.
Therefore, prior workers have concentrated principally on recovering the metallic aluminum content of dross while letting the aluminum oxide content of the dross go to waste. U.S. Pat. No. 3,770,424 can be considered typical. Greater amounts of metallic aluminum may be recovered from the dross if it is fluxed with ample amounts of chlorine or other halide fluxes, as illustrated in the very early U.S. Pat. Nos. 1,180,435 and 2,768,075, and, more recently, in 3,676,105, 3,751,243 and 3,694,190. While such processes tend to reduce the amounts of carbides and nitrides which contaminate the alumina, they increase the total impurities in the alumina by amounts of halide fluxes entrained in the alumina, so such alumina is unsuitable for making primary aluminum in Hall cells.
Where dross is not contaminated with chloride fluxes, as when dross is recovered by skimming crucibles of molten aluminum in the potrooms, the alumina as well as entrained metal in the dross may be recovered by steam treatment, as disclosed in U.S. Pat. No. 3,660,076. The steam decomposes the nitrides and carbides, so the recovered alumina and entrained aluminum are suitable for adding back to the reduction cells.
It has long been known that alumina in activated form would sorb various gases such as sulfurous gases, but means have not been available to remove such sulfurous components completely enough, and recover them as a byproduct, in an economic fashion. Likewise, aluminous materials have been used as fluxes in steel manufacture, but results have not been uniformly good enough for the steel industry to adopt alumina as a substitute for fluorspar in routine commercial operations.